1. Field of Invention
The invention relates to a digital data transmission system comprising a data transmission network formed by several node stations and interconnecting high-speed trunk lines, each node station comprising several low-speed parallel subscriber interfaces controlled by a network management system, whereby subscriber stations are connectable to the subscriber interfaces either directly or through a serial modem link, depending on the distance.
2. Discussion of Related Art
Developments in technology are rapidly leading to digital telecommunication networks. In Europe, standards have been defined for telephone network facilities to ensure the compatibility of the devices of different manufacturers. These standards concern frame structures used in multiplexing and electric interfaces of the systems and facilities. They are based on 2M basic multiplexing the speech channels of which are 64 kbit/s digital channels to be multiplexed to the frame structure in a predetermined standard way. Thirty time slots of the frame structure are reserved for the speech channels and two for other data transmission purposes and synchronization. These devices are typically used to establish connections between exchanges.
As data transmission has become more general, speech and data have began to be transmitted through the same system. Different manufacturers have developed various interface units to provide a data link to a telecommunication network. Since the basic rate in speech multiplexing is 64 kbit/s and the typical data transmission rates are 1,200, 2,400, 4,800, 9,600 and 1,9200 bit/s, it has become necessary to develop a certain subchannelling by means of which a single 64 k channel is provided with several data transmission channels. For this channelling a number of international standards have been introduced, such as the X.50, X.58 and V.110 specifications. The data transmission interface also uses specific standards, the most widely used being the V.24/V.28 specification.
Rapid increase in the amount of data transferred through a telecommunications network has made it economically feasible to construct an independent network solely for data transmission purposes to serve a city or for national service systems, for instance. This kind of network aims mainly at satisfying the need of fixed data transmission connections, but these typically have to be altered frequently (such as changes of transmission rate). It has, in fact, been estimated that alterations are required in a fixed data transmission connection about once in two years.
When the number of fixed connections increase up to a certain limit, the construction and alterations of the connections become too laborious. Network structures enabling flexible establishment of connections between two points have been developed. For this purpose, the network has to be provided with cross-connection means, which switch the channels of the standard frame in a desired manner, and network management facilities control the switching operations of the network and, e.g., changes of transmission rate. In this way very large networks can be formed and nevertheless maintain the manageability of the network.
Since standardization has not concerned the network management, different manufacturers have their own specific systems. The network can be controlled from one or more points. In general, the network management system is able to monitor and control also the individual parallel subscriber interfaces of the nodes which connect the subscriber devices to the network, and, as a consequence, alter the interface configuration, transmission rate or various signal delays in a required manner. When a subscriber device is connected directly to a subscriber interface, the network can be monitored and controlled completely up to the subscriber devices.
In many cases, particularly with services supplied by a telecom office, the node of the network, comprising the cross-connection means, multiplexing means and subscriber interfaces, is located in the premises of the office in a centralized manner while the subscriber facilities are located in the customers' premises within a radius of a few kilometers from the node station. A serial transmission link is thereby established between the subscriber interface and the subscriber device by means of modems, which provides relatively long transmission distances. As the length of the link is usually only a few kilometers, relatively simple baseband modems can be used.
A problem with a modem link is how to bring it under the control of the network management system. Even though the network management system in its present form is able to control the subscriber interface, this is of no use if the system is not able to affect the configuration of the modem link, such as the transmission rate. Centralized supervision of the condition of the modem link is not possible either.
The problem is further aggravated by the great variety of digital networks and different management systems used by different manufacturers and in different networks, as there exists no standards. In most cases, modems have different manufacturers than the other network facilities, and the modem manufacturers have to match their modems with the different network management systems. This is, of course, possible but uneconomical from the viewpoint of the modem manufacturers on account of the resultant small production series and the great number of different models. On the other hand, the manufacturer of the data transmission network has to provide the network management system with programs supporting the modems.
The connection of modems to standard interfaces and network management systems of various manufacturers is equally problematic in packet switched networks.